This invention relates to cryogenic fluid vaporizers and particularly to vaporizers heated by exposure to ambient atmosphere.
Atmospheric gases produced by separation of air, such as oxygen, nitrogen and argon, find wide use in a variety of industrial applications. Large quantity users of such a gas, such as steel mills or aluminum remelters, may have air separation plants installed at the usage site. Small quantity users of such gases typically purchase the quantities required in high pressure cylinders. Intermediate quantity users of such a gas typically find it convenient to purchase a supply of the gas in liquid form, that is, as a cryogenic liquid, maintain it in a storage tank at the usage site and vaporize the cryogenic liquid from the tank as needed in a vaporizer. Cryogenic liquid as used herein is defined to mean a liquid boiling at temperatures below 200K.
A user may require intermittent or continuous flow of gas to be generated from a cryogenic liquid stored in a tank. To produce a continuous flow of gas by vaporizing liquid, a heat exchanger may be used with heat supplied by a hot fluid, such as steam generated in another process. Alternately, an electrical heater may be employed. However, the most common source of heat for continuous and intermittent cryogenic liquid vaporization is the ambient atmosphere.
An atmospheric vaporizer system is typically comprised of one or more passes of tubes or modules vertically positioned. The exterior of the tubes is exposed to the ambient atmosphere and may have extended surface. The cryogenic liquid is caused to flow in the interior of the tube where it is vaporized and is superheated as required--perhaps even to approach the ambient atmospheric temperature.
As the cryogenic liquid passes through the atmospheric vaporizer system, the exterior surfaces of the vaporizer system are cooled. The exterior surfaces of a conventional ambient vaporizer system typically range from temperatures approaching the boiling temperature of the cryogenic fluid, such as 77K for nitrogen to temperatures approaching the ambient air temperature. The cold exterior surfaces of the vaporizer system cool the surrounding air. When the temperature of the surrounding air is cooled below its dew point, a film of water is deposited on the exterior surfaces of the vaporizer system and a mist of condensed water, that is, fog, is formed in the air. On the portion of the exterior surface which is below the freezing point of water, the water freezes and ice builds up over time. The ice build up may completely fill the space between adjacent fins on the exterior of the vaporizer tubes, and, in time, may even fill the space between adjacent tubes. Ice build up presents several problems. It reduces the surface area of the vaporizer and acts as an insulation. Both effects decrease the rate of heat transfer from the ambient atmosphere to the exterior surfaces of the vaporizer and thus the capacity of the vaporizer. The ice may build up to a weight ten or more times greater than the weight of the vaporizer itself. The structure of the ice is not uniform, nor predictable. Portions of ice may spall off intermittently during operation, or during deicing maneuvers, presenting a hazard to the vaporizer itself, associated piping and attendant personnel. Furthermore, the fog generated in the vicinity presents a hazard to vehicular and pedestrian traffic due to reduced visibility.
Management of the problem of ice build up has been attempted in several ways. Periodic manual deicing is performed by personnel by applying external hot water jets or steam jets, and by mechanical removal using picks and shovels. The practice is undesirable in that manual action is required. The ice structure is unpredictable. Falling ice may injure personnel performing the work and may structurally damage the vaporizer and associated piping. Fog generation is not reduced by such manual, periodic deicing.
A management technique is to accommodate ice build up on an initial length of bare piping, that is, piping without external finning. The bare piping is then followed in series by piping with external finning. The bare piping is intended to provide most or all of the surface for ice deposit. The logic is that the bare piping is less costly than the finned piping and can be supported in a less costly array to accommodate high ice build up. However, an undesirably large amount of bare piping, floor space, and structural support needs to be used, making this approach unattractive. Fog generation also remains a problem and is not reduced by this technique.
Another approach has been to provide one or more duplicate banks of vaporizers. While one bank is in active service, one or more other banks may be defrosting. A number of schemes may be used for switching banks. A simple scheme is to switch banks purely on a time schedule thereby disregarding other considerations. This approach uses redundant vaporizers which are expensive and also increase space requirements. Fog generation, however, remains a problem and is not reduced by this technique.
Yet another approach has been to oversize the vaporizer system resulting in reduced average heat transfer loading per vaporizer module, thereby increasing the cost and floor space requirement. Fog generation usually is reduced somewhat by this technique.
For the foregoing reasons, there has been a need for a vaporizer system for cryogenic liquids which eliminates or reduces icing of the exterior surfaces of the vaporizer which are exposed to the ambient atmosphere, and reduces fog generation without requiring excessive redundant vaporizer surface area or vaporizer structure.